This invention relates generally to systems for monitoring physical properties and, more particularly, to a method and apparatus for determining statistical characteristics of the physical properties of material, such as the sheet material, as it is being produced.
While the present invention has a large variety of applications, it is particularly applicable in the production of paper and, hence, will be described with reference to paper manufacture. Upon review of the following description, however, it will be appreciated that the present invention may be used in many other situations in which a varying physical property is monitored. In many such situations, it is not feasible to sample, store, and analyse the physical property values at a sample rate sufficient to reflect the fluctuations in the property. The present invention provides a practical way of developing meaningful statistical indicia relating to the property without storing all of the sampled values.
It is well known in the prior art to monitor characteristics of a paper web as it is being produced to determine and control the averages of those characteristics. For example, moisture, basis weight and caliper may be monitored and the resulting signals utilized in feedback control systems to maintain the averages of the properties within required specification limits.
While such process control systems are known in the prior art and perform well in maintaining the averages of a variety of properties of the paper web within desired specifications, peak variations in the properties still occur during the paper manufacturing process. Since paper is graded according to the monitored properties of the paper web and the deviations of those properties from defined specifications, it is important to be able to quantify the properties and their deviations to classify the paper for further processing and sale.
As paper is manufactured, it is formed into rolls which are oftentimes immediately transferred from the paper making machine to the next processing station where the rolls are cut into desired sheet sizes and packaged for shipment to fill orders requiring various specifications. Accordingly, it is important to be able to monitor and analyze the properties of the paper web rapidly to prevent delays in the manufacturing process.
In the prior art, the properties of a roll of paper are normally averaged over segments of paper tens of feet long due to the paper machine operating speeds and the time constants of the monitoring gauges. Since the paper is to be cut into small sheets, for example 81/2 inches by 11 inches, the averaging of the characteristics of the paper over long segments overlooks individual discrepancies or fluctuations in the properties which may appear in one or many of the sheets which are cut from the paper roll.
It is thus apparent that a need exists for an improved method and apparatus for determining statistical characteristics of measurable properties of sheet material as the sheet material is being produced so that timely decisions can be made as to the classification and appropriate further processing of the sheet material.